1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus which is adapted to radiate a plurality of beams onto an information recording medium for recording information optically and reproducing the information just recorded.
2. Description of the Related Art
An optical information recording and reproducing apparatus can optically record and reproduce much more information concentrically or spirally on and from a rotating disk-shaped information recording medium with a higher density than a conventional apparatus employing a magnetic disk. Such an optical recording and reproducing apparatus therefore has the advantage that it can provide a greater recording capacity. However, an information recording medium employed in an optical information recording and reproducing apparatus encounters defects more often than a magnetic disk medium does, thus making it necessary to provide the apparatus with a function of maintaining the reliability of recorded information. To meet this requirement, a two-beam optical head has been proposed. This optical head uses a recording beam and a reproducing beam each having a different wavelength. These beams each form a spot, and the recording beam spot is used in recording whereas the reproducing beam spot is used in reproduction immediately after the recording.
FIG. 15 illustrates a conventional two-beam optical head, which is shown in, for instance, Theses at 15th Microoptical Meeting (pages 58 to 62). In the drawing, a recording semiconductor laser 1 radiates a recording beam 2 (indicated by broken lines) which has a wavelength .lambda.1 (e.g., 830 nm), while a reproduction semiconductor laser 3 radiates a reproducing beam 4 (indicated by solid lines) which has a wavelength .lambda.2 (e.g., 780 nm). The beams 2 and 4 each become a parallel beam after passing through a collimator lens 5 or 6. A triangular prism 7 is provided to change the cross-sectional configuration of the recording beam 2 into a generally oval shape. An interference filter 8 acts to allow a beam of the wavelength .lambda.2 to pass therethrough while reflecting beams of other wavelengths. A dichroic prism 9 acts to allow a beam of the wavelength .lambda.2 to pass therethrough while reflecting a beam of the wavelength .lambda.1. A polarized beam splitter 10 is disposed in such a manner as to face the interference filter 8 and the dichroic prism 9. An objective 12 faces the splitter 10 with a one-quarter wave plate 11 therebetween, and an information recording medium 13 faces the objective 12. The recording beam 2 converges on the information recording medium 13 to form a recording spot 14, while the reproducing beam 4 converges on the medium 13 to form a reproducing spot 15. A detection optical system 16 is provided for receiving and detecting the reproducing beam 4 reflected by the information recording medium 13, and the system 16 comprises a convergent lens 17, a cylindrical lens 18, and a photodetector 19. Another photodetector 20 acts to receive and detect the recording beam 2 reflected by the information recording medium 13, and it is provided for the purposes of, for instance, detecting whether the radiation of the recording semiconductor laser 1 is correct or not.
The functions provided by the detection optical system 16 include detecting a reproduction signal and servo signals (i.e., a focusing error signal, and a tracking error signal).
In the optical head having the above-described structure, the recording beam 2 of the wavelength .lambda.1 is radiated from the recording semiconductor laser 1, is reflected by the interference filter 8, and is then transmitted through the polarized beam splitter 10 since the beam 2 comprises components which are all p-polarized components with respect to the splitter 10. Thereafter, the recording beam 2 forms a recording spot 14 on the information recording medium 13 so as to record information. The recording beam 2 reflected by the information recording medium 13 transists the one-quarter wave plate 11 twice while it makes back-and-forth trips between the medium 13 and the polarized beam splitter 10, whereby its direction of polarization is rotated through 90 degrees so that the beam 2 becomes an s-polarized beam. The beam 2 is therefore reflected by the polarized beam splitter 10, and it is further reflected by the dichroic prism 9, finally being received by the photodetector 20. On the other hand, the reproducing beam 4 of the wavelength .lambda.2 is radiated from the reproducing semiconductor laser 3, passes through the dichroic prism 9, and is reflected by the polarized beam splitter 10 since the beam 4 comprises components which are all s-polarized components with respect to the splitter 10. Thereafter, the reproducing beam 4 forms a reproducing spot 15 on the information recording medium 13 so as to reproduce the information. The reproducing beam 4 reflected by the information recording medium 13 transists the one-quarter wave plate 11 twice while it makes back-and-forth trips between the medium 13 and the polarized beam splitter 10, whereby its direction of polarization is rotated through 90 degrees so that the beam 4 becomes a p-polarized beam. The beam 4 is therefore transmitted through the polarized beam splitter 10 and its transmitted through the interference filter 8, finally entering the detection optical system 16.
FIG. 16 illustrates the relationship between the recording and reproducing spots and the information recording medium. In this drawing, it is assumed that the information recording medium 13 rotates in the direction of an arrow J. Information is recorded as pits 21 which are each formed in the medium 13 by the recording spot 14. The reproducing spot 15 is positioned at a distance l behind the recording spot 14, and the pits 21 which have just been recorded are sequentially reproduced. In this way, information can be reproduced as the recording proceeds.
The conventional two-beam head having the abovedescribed arrangement encounters the following problems. First, in order to detect reproduction signals with a high level of sensitivity, it is necessary to use two semiconductor lasers to produce beams, i.e., recording and reproducing beams, with a large difference in wavelength therebetween. Secondly, the optical elements used for discriminating beams by difference in wavelength, such as the dichroic prism and the interference filter, must have strict wavelength discriminating characteristics, in addition to the requirement that the semiconductor lasers must generate beams whose fluctuations in wavelength are sufficiently small. Thirdly, since the same one-quarter wave plate is used for the beams of the two different wavelengths, the direction of the polarization of the recording or reproducing beam cannot be completely rotated through 90 degrees. As a result, the separation of the beams by the polarized beam splitter cannot be complete, adversely affecting the level of resultant signals.